1. Field of the Invention
The present invention relates to a refrigerating circuit for car air conditioning, and particularly, to an improvement of a refrigerating circuit having a continuously variable displacement refrigerant compressor.
2. Description of the Related Art
Generally, in a refrigerating circuit for car air conditioning, a refrigerant compressor is intermittently driven by an engine of the car to cope with changes in the cooling load of a car compartment. This intermittent operation, however, causes fluctuation of the torque of the engine, which leads to an uneven drive of the car and causes wear of a clutch disposed between the compressor and the engine. To solve this problem, a variable control of the displacement of the compressor has been proposed.
Two types of variable displacement mechanisms for the compressor are known, i.e., a stepwise variable type that selectively changes the displacement, and a continuously variable type that continuously changes the displacement, and it has been proposed that these mechanisms be used for the refrigerating circuit for car air conditioning.
FIG. 3 is a schematic view showing a conventional refrigerating circuit. In the figure, 10A denotes a compressor driven by, for example, a car engine, and this compressor 10A is produced with a discharge conduit 12 from which a refrigerant circuit is extended. Disposed in series in the refrigerant circuit are, a condenser 14, a refrigerant receiver 16, an expansion valve 18A, and an evaporator 20. A circuit comprising a suction conduit 22 extends from the evaporator 20 to the compressor 10A. The expansion 18A is a conventional thermostatic automatic expansion valve.
An equalizing pipe 24a transfers a refrigerant pressure at an outlet of the evaporator 20 to the expansion valve 18A, and the transferred refrigerant pressure acts on a spring of the expansion valve 18A to generate a resultant force. The resultant force is balanced in the expansion valve 18A by a saturation pressure of a thermosensitive cylinder 24b responsive to a refrigerant temperature at the outlet of the evaporator 20, to thereby adjust an opening of the expansion valve 18A. The expansion valve 18A maintains a superheating temperature of a suction refrigerant of the compressor 10A at a fixed value (about 10.degree. C.).
The compressor 10A is provided with a continuously variable displacement mechanism and a valve device 10a. The valve device 10a detects a suction pressure of the compressor 10A and adjusts a discharge displacement of the compressor 10A accordingly, to thereby control the suction pressure to a constant value (about 2 atm).
In a typical refrigerating circuit for car air conditioning, the refrigerant compressor is arranged in the engine room, and therefore, contrary to an evaporator disposed in a passenger compartment, the compressor is greatly affected by heat. Particularly, in a front-engine front-drive (FF) car, a suction conduit must be extended from the evaporator located in the passenger compartment to the compressor located in front of an engine of the car, and this type of structure not only causes the compressor to be affected by heat but also causes a fluctuation of a flow of refrigerant passing through the suction conduit.
Therefore, even if a superheating temperature of the refrigerant at an outlet of the evaporator is controlled by an expansion valve of the type described above, external factors such as heat have an adverse influence on the suction conduit and cause substantial changes in the superheating temperature of the refrigerant to be sucked by the compressor. As a result, the temperature of the discharged refrigerant may be increased, and a cylinder of the compressor may be abnormally overheated.
The expansion valve does not control an evaporating temperature to a constant value but maintains a pressure difference between an evaporating pressure and a saturation pressure corresponding to the temperature of the refrigerant at the outlet of the evaporator, at a constant value. Therefore, the evaporating temperature (a temperature of conditioned air) cannot be stabilized, and further, part of the evaporator is always under a refrigerant superheated state and thus a cooling capacity of the refrigerating circuit is lowered.